Monocrystalline materials may offer a high degree of uniformity in terms of their physical properties as well as a high degree of repeatability and efficiency when compared to polycrystalline ceramic materials (polycrystals) of the same material. Therefore, monocrystals or single crystals which do in fact exhibit such properties are highly prized as replacements for polycrystalline materials wherever such materials are used. Moreover, because of their highly desirable properties, monocrystalline materials are sought out for applications for which polycrystalline material would never be considered. For example, in optical applications, polycrystals will provide a diffuse reflection of incident light whereas monocrystals would yield specular reflection.
In accordance with the present invention, and indeed generally, monocrystals can be differentiated from polycrystals based on a number of factors. Monocrystals are sized and shaped such that they can be used individually in the production of sensors, probes and the like. Polycrystalline materials are those made up of a composite of many individual crystals. Many ceramic materials are polycrystalline in nature as are many rocks and fabricated metals. The size of crystallites in polycrystalline material are usually small with equivalent diameters in many materials varying from a few micrometers to about 100 micrometers. Much larger crystallites are possible. However, because of physical properties such as, for example, packing density and other issues common in the ceramics field, it is usually advantageous to insure that the individual crystals do not get too large and are of as uniform a size and composition as possible. In the context of the present invention, however, when referring to a single crystal, preferably, an isolated crystal which is large enough to individually manipulate is envisioned. Preferably, individual crystals are desirably at least about a millimeter in size or greater, along one edge. Crystals of a centimeter on a single edge are desirable for many applications. In addition, significant improvements in precision (repeatability) and accuracy (true value) are achieved when monocrystals are diced into specimens or slices of the order of, or less than, a few tenths of a millimeter size along an edge.
There are other significant differences as well, most of which stem directly from the fact that monocrystals are, as their name implies, singular, whereas polycrystals involve the interaction of at least two crystals and suffer from charge carrier scattering at their grain boundaries, where modifications to the electric conductivity likely takes place. In fact, transport processes such as electric and thermal conductivities and dielectric properties of the crystal differ considerably from those of ceramic samples.
Polycrystalline ceramic materials (polycrystals) can be thought of as a composite material made up of two or more distinct individual crystals but usually a large number of crystals. Just as the polycrystalline materials are composites of the individual crystals, so too are their properties. Polycrystals have voids and often two or more stoichiometries and phases. These features each have an effect on the overall properties of the material and any device or sensor made using them. For example, the dielectric tensor of semiconducting polycrystalline materials is less than that of the corresponding semiconducting monocrystal of that same material because of the presence of voids. The resistivity (inverse electric conductivity) and the thermal conductivity of the polycrystalline material is also affected thereby. Monocrystals, which do not suffer from such composite properties will not exhibit such a strong dispersion in their impedance-frequency characteristics. See E. G. Larson, R. J. Arnott, D. G. Wickham, "Preparation, Semiconduction and Low Temperature Magnetization of the System Ni.sub.1-x Mn.sub.2+x O.sub.4 ", J. Physics and Chemistry Solids, Vol.23, (1962), 1771-1781..sup.(1) See also Oxide Magnetic Materials, K. J. Standley, Monographs on the Physics and Chemistry of Materials. 2 ed. Clarendon Press. Oxford (1972), pp. 140-141..sup.(2)
Moreover, since no two groups of polycrystals can be exactly the same, i.e., same number of crystals of identical size, orientation, stoichiometry and composite properties, even within the same lot, the response of one sensor made with one group of polycrystals may vary with respect to other such sensors. Polycrystals may also be problematic because they may absorb water, particularly in the voids between crystals. When such material is exposed to variations in humidity, "aging" or a lack of reproducibility of properties over time and/or temperature may be accelerated and greater in comparison to comparable monocrystals. Moreover, the size of the voids between individual crystals may change with time and exposure to the elements and in response to external electric fields. Again, the thermal and electrical properties of the resulting material may therefore change over time. Monocrystals do not suffer from these same aging limitations.
Finally, with regard to certain materials and in particular cubic spinel crystals, there may be magnetic ordering effects over temperature in monocrystals. But with polycrystals, exposure to a magnetic field may cause movement of the individual crystals. This would result in a change in the grain boundaries and the size and shape of any voids and lead to hysteretic effects. The composite properties of the material would change accordingly. Certainly, the magnetic, thermal and electric properties of monocrystals can be more accurately measured and more repeatedly relied upon than polycrystalline materials.
Cubic spinel crystals, such as the crystals of the present invention, provide isotropic properties. For example, transport processes, such as electric conductivity, are isotropic.
Polycrystals of nickel manganese oxide are known, see D. G. Wickham, "Solid-Phase Equilibria In The System NiO--Mn.sub.2 O.sub.3 --O.sub.2," J. Inorg. Nucl. Chem., 26, (1964), 1369-1377.sup.(3), as are monocrystals of this system. See Rosen et al., U.S. Pat. No. 5,653,954, the text of which is hereby incorporated by reference and attached. Polycrystals of nickel-cobalt-manganese are also known to exist. For example, polycrystalline materials of nickel-cobalt-manganese oxides were reported by L. V. Azaroff, see Z. Kristallogr., volume 112, pages 33-43, (1959). However, monocrystals of nickel-cobalt-manganese were unknown until their invention by the present inventors. See Rosen et al., U.S. patent application Ser. No. 08/877,415, filed Jun. 17, 1997 and titled Growth of Nickel-Cobalt-Manganese Oxide Single Crystals, the text of which is hereby incorporated by reference.
Despite the success the inventors achieved throughout the growth of nickel-manganese oxide and manganese-cobalt-nickel oxide monocrystals, there was no way to predict how the introduction of an additional variable to the system (namely iron) would affect the ability to produce monocrystals or their comparative properties to the corresponding polycrystals. There is simply no way to predict what the result would be. This is particularly true in this case because of the peculiar chemistry of iron with respect to its oxidation states.